1. Field of Invention
This invention relates to aerodynamic surface controls, and more particularly to provision of forces for feel augmenting, restoring and positioning the control lever thereof in response to loading thereon.
2. Description of the Prior Art
The positioning of aerodynamic surfaces in response to pilot-actuated controls has taken many forms, from simple cable connections to complex electronically augmented hydromechanical systems. For instance, in helicopters, it is well known to provide an outer control loop in which pilot-manipulated controls provide pitch-adjustments to a main rotor and stabilizing rotor, with an electronic inner control loop responsive to inertial measurement equipment on the helicopter for providing inertial-stability inputs to control the same surfaces in a fashion tending to stabilize the helicopter in flight. This type of system lends itself to the use of hydromechanical actuators for control surface positioning, the pilot-actuated controls being buffered from the effects at the control surfaces by the hydromechanical system. As is commonly known in the case of power steering in an automobile, this takes the "feel" out of control manipulation; therefore, it has been known to provide feel augmentation systems in which forces are purposefully applied back to the pilot-actuated controls in a manner related in some fashion to the surface being controlled thereby, to impart information to the pilot through the feel of the control itself.
In the case of helicopter rotors, the principle means of maneuvering the craft is to control the pitch of the rotor blades collectively (all in the same fashion) to cause the lift to vary, thereby imparting altitude changes, as well as cyclically (as the function of the position of each rotor blade with respect to the aircraft as the rotor rotates) in order to control pitch and roll, thereby to control turns and forward speed. It was soon learned that causing too great a pitch angle of the rotor blades induced vibration in the rotor, which could be heard by the pilot as well as felt in the collective pitch control, since a small fraction of the vibration was transmitted through the collective pitch actuating mechanism to the control lever itself. Thus, by the vibration felt in the lever as well as the sound induced by vibration, the pilot could maintain a collective pitch which was just below that causing vibration to ensure that the rotor blades were not subjected to excessive aerodynamic loading. With improvements, the vibration in the rotor as a result of blade loading has been significantly reduced. This results in a loss of the vibration-indication of rotor blade loading. As a substitute, it is known to measure the force required for the collective pitch actuating mechanism to move the swash plate (which alters the pitch of the blades) by means of an electromechanical transducer, which provides an indication of rotor loading on a cockpit panel instrument, called a cruise guide indicator. However, during maneuvering of the helicopter, visual concentration on the cruise guide indicator interferes with the pilot's ability to make visual observations of the environment, and presents potential safety hazards.